1. Field of the Invention
The present invention is directed to a container assembly for use in the sampling and growth of microorganisms. More particularly, the invention is directed to a container assembly having a base and a lid where the lid is removably coupled to the base by a locking mechanism.
2. Discussion of the Related Art
Containers for culturing microorganisms are commonly referred to as petri dishes or contact plates and are well known in the art. These devices typically include a bottom container or base or dish portion for the growth medium and a peripheral side wall. The bottom container is typically used in conjunction with a lid having a top wall and a peripheral side wall that is able to fit over the side wall of the bottom container. Generally, the side wall of the lid forms a loose fit with the bottom container to prevent the lid from binding with the bottom container. A contact plate is a much smaller version of a Petri dish, the dish component of which is pre-loaded with growth medium and is provided with a base for grasping the contact plate so as to permit it to be pressed against a surface to obtain a sample of any microorganism(s) present on the sampled surface. Containers such as these are manufactured by several manufacturers and are readily available.
Contact plates and petri dishes are typically fabricated from polymeric material in mass quantities at a sufficiently low cost as to be disposable after a single use. The dish portion of the contact plate is filled with a generally convex mound of growth medium. With the lid removed, the contact plate is grasped by its base and the mound of growth medium is pressed against a surface to be tested for bacterial and/or fungal contamination. The lid is then replaced and the contact plate is stored in an environment conducive to microorganism growth. A typical contact plate is pre-loaded with growth medium under sterile conditions and packaged for shipment to the end user.
Contact plates and petri dishes for microorganisms are commonly used in large numbers each day by a technician. A technician can use several hundred containers each day to test for various microorganisms. To enable the technician to handle the large number of containers efficiently, the lids must be easily removed and replaced without sticking or binding together. In some instances, it is desirable to have the lid sufficiently loose to enable the technician to remove the lid with one hand while dispensing the sample into the petri dish for testing. Other uses of the petri dish require that the lid be secured to the base to prevent inadvertent separation of the lid from the bottom container but still allow the lid to be removed from the dish with minimal effort.
One problem that often occurs with petri dishes and contact plates during use is the risk of the technician disrupting the growth media after the sample has been taken. This typically results in inaccurate sampling results since unwanted microorganisms are often transferred to the growth media by the inadvertent touching or separation of the lid from the base exposing the growth media to an unintended environment. To reduce the risk of inadvertent contact with the growth media, various methods have been proposed for securing the lid to the base container to prevent inadvertent separation of the lid from the base container. These efforts have often resulted in high frictional forces that can make it difficult to separate the lid from the container. For example, two known designs of contact plates are those that are the subject of U.S. Pat. Nos. 5,854,065 and 6,602,704. Both designs provide a locking mechanism in which the lid and base are held together by a compression fit that may be either too tight to allow ready disengagement between the lid and base container or too loose, which can lead to inadvertent separation of the lid from the base container when handling the contact plate.
Another approach for securing the lid to the base container is the subject of U.S. Pat. Nos. 6,969,606 and 6,969,607 and 7,452,711 which disclose a lockable contact plate wherein the locking members consist of pairs of radial sheaths and tabs. However this approach requires a rotational movement (with a torsional force) of the lid with respect to the base in order to lock or unlock the locking members. A repeated rotational movement using a torsional force can have ergonomic issues for users such as technicians who can handle several hundred containers each day. In addition a rotational locking mechanism can inherently be over torqued/tightened when locked such that an excessive torque force is required to open the locking members.
What is needed therefore is a lockable Petri dish or a lockable contact plate that does not lock except upon application of a specific intentionally applied compressive force, that cannot be over torqued, that provides a secure locking engagement between the lid and the dish, and which may be readily disengaged from the locking engagement without the need for a rotational movement or torsional force.